1. Field of the Invention
This invention relates to mufflers for air-cooling systems used for channeling air for cooling heat-generating apparatus and, more particularly, to a novel and highly effective muffler for an air-cooling system used for cooling electric motors, the muffler enabling the system to be operated at a very low perceived noise level.
2. Description of the Prior Art
Electric motors have been developed for many different applications and markets around the world. Over the years, intensive efforts have been made to standardize production and incorporate all the best features into one motor line.
Efficient cooling of electrical apparatus such as electric motors is essential to ensure long life. Electric motors are normally air cooled. For example, the rotor may incorporate blades that cause air to circulate around the motor windings and bearings to dissipate excess heat. In large units, a separate fan may be provided for the forced circulation of air. In either case, means may be employed in order to channel the cooling air so that it flows in close proximity to the motor and in particular to the portions thereof that require the most cooling.
Noise suppression is also important. Electric motors are used in factories and other locations in close proximity to workers, customers, and the general public. Many studies have shown that excessive noise levels can lead to irritability, elevated blood pressure (hypertension) and hearing loss. In order to increase worker satisfaction, improve worker productivity, and in general create a more pleasant environment for workers, customers and the public, it is desirable to make electric motors run as quietly as possible.
The goals of cooling and noise suppression are to some extent contradictory. Efficient cooling necessitates channeling a flow of air into close proximity to various parts of the motor, which leads to a rather labyrinthine air path. The repeated abrupt turns of the air as it moves through the electric motor set up a spectrum of noise within the audio range. In addition, the operation of the motor creates a main discrete noise component as explained below. It is difficult to suppress noise throughout the audio spectrum since measures taken to suppress high-frequency noise may not be very effective in suppressing low-frequency noise and vice versa. Moreover, measures taken to improve cooling efficiency may increase the noise level.
A number of attempts have been made in the past to provide efficient cooling while suppressing excessive noise. A U.S. patent to Picozzi et al. U.S. Pat. No. 2,886,721 discloses a motor enclosure which is lined with a mastic damping compound and has a fiberglass blanket on the damping compound and a wire mesh superposed on the fiberglass blanket. The motor enclosure includes internal baffling. In operation, air is drawn downwardly through louvers provided in the side of a muffler and passed inwardly through openings provided in the bottom of plenum chambers and into a fan. As the rotor revolves, the fan generates an air pressure that directs the incoming air through the stator and then to the muffler unit where the noises generated during its path of travel through the motor are absorbed by the insulation in the muffler unit. As the air enters the muffler unit, it is caused to travel through a tortuous path prior to ejection from the casing.
A patent to Jaeschke No. U.S. Pat. No. 3,389,287 discloses a structure wherein air is drawn into and through inlet passages and impelled through a plenum chamber by a finned rotating spider and inductor drum. The air is then driven out through outlet passages. The sound to be suppressed passes out from the plenum chamber through the inlet and outlet passages. Thus the sound is propagated oppositely to the inlet flow of air and in the same direction as the flow of exhaust air. The wavelength of the sound as it travels in these portions is calculated, and chambers are provided the effective inside lengths of which are about equal to the wavelength peaks at about 870 and 1300 Hz. This is intended to cancel noise at or near 870 Hz and 1300 Hz.
A patent to Sakurai No. U.S. Pat. No. 4,119,873 discloses an electric motor wherein cooling air taken from the outside of the machine is circulated through a housing of the machine by means of a fan, and a casing containing slip rings is mounted on the outside of the housing. A portion of the cooling air pressurized by the fan is circulated through the casing for cooling the slip rings.
Other patents of interest are a patent to Falk No. U.S. Pat. No. 771,468, a patent to Curzon U.S. Pat. No. 2,505,854, a patent to Lewis U.S. Pat. No. 2,726,807, a patent to Egglestone et al. U.S. Pat. No. 2,777,079, a patent to Panza U.S. Pat. No. 3,980,912, a patent to Lawyer et al. U.S. Pat. No. 4,007,388 and a patent to Mayher U.S. Pat. No. 4,230,960.
None of the prior art represents a complete solution to the problem of efficient cooling of electrical apparatus or other heat-generating apparatus combined with maximum suppression of the noise generated by the apparatus and by the cooling system itself.